1. Concerning boundary value problems;Neuberger, J. W.;Pacific J. Math.,1960
2. Concerning harmonic matrices;Wall, H. S.;Arch. Math. (Basel),1954
3. Adjoint linear differential operators;Zettl, Anton;Proc. Amer. Math. Soc.,1965
4. \bysame, Related two-point boundary value problems and harmonic matrices, Doctoral Dissertation, University of Tennessee, Knoxville, 1964.